Not since the Tron debuted have I spent this much time thinking about cars that don’t exist that I want to drive. With the increasing synergy between design and practical engineering, and given the attention to global warming and peak oil, major and minor automakers are stepping up and taking notice. And they’re doing it in style.

A handful of sexy alternative vehicles have already appeared or will soon make it to market – see our top 5 near-term sexy “G” production vehicles here – but that leaves out a vast swath of dreamers imagining a better, sexier way.

So here they are, with less ado then they deserve – the top 5 sexy concept cars.

5. Audi R-Zero
Admittedly this puppy looks like an ethereal fantasy, and will likely remain just that. In other words, what better way to start the list!

The numbers buzzing around this concept are highly questionable – I imagine the R-Zero’s French design school creators snickering over their laptops and chatting with some silly, silly blogger – but Audi concepts our in-wheel motors would give this car a top speed of 286mph, 1091 horsepower, and 0-62 in 3 seconds. 62? What, 60 is too pedestrian? Still, quite sexy.

4. Citroen 2CV
The best concepts start as rumors, and the restyled Citroen 2CV is just that.

According to our sources (read: myself and Google) this modern take on the iconic French runabout may appear as early as 2009. If and when it does, it’s said to incorporate a diesel-electric hybrid powerplant with plastic body panels – corn anyone? – comprising its curvaceous form. The 2CV’s unforgettable and unmistakable lines, sexy heritage, not to mention my imagined first sighting of it set on the streets of Paris (Oui, oui mademoiselle!) are all enough to eek out a spot on the list.

3. Chevrolet Volt
Few concepts are as hyped as this repentant offering from what used to be the world’s largest automaker. Had GM not scrapped the EV-1 blah-blah-blah — you’ve heard it all before…

In any case, the Volt may be it’s last, best chance to bite the bullet and evolve with the times. An onboard diesel, ethanol or gasoline generator will supplement a pluggable, all-electric drive train, an efficient configuration that will be adopted by GM’s Opel badge across the pond. Borrowing masculine styling cues from its Lincoln line, GM is hoping that Detroit’s faithful will finally grasp the statistical reality that they commute less than 40 miles a day, and will want to do so primarily emissions-free. 60,000 Volts are expected in the first model year, so if the world isn’t slightly more sexy come 2010, you’ll know who did it. Again.

Running off B100 ethanol (We know, we know; ethanol will steal your food), the 400-horsepower V6 would hurtle this Scandinavian beauty to 60mph in a decent and realistic-sounding 4.9 seconds. There’s no word on whether the born-from-jets roof door combo will feature explosive bolts for the inevitable premature ejections to follow its unlikely release, but this may well be the sexiest “G”-rated Swedish export the world sees in a long time. OK, PG-rated.

1. VentureOne
Although the makers of this three-wheel two seater have said they expect the first VentureOne to roll nimbly off the production line sometime in ‘08, this bad boy remains squarely in the concept car category.

If things go as planned, this motorcycle-by-definition will arrive in three flavors. Two hybrids — the standard e50 and high-performance Q100 — are engineered not unlike the Chevy Volt, with onboard gas generators powering in-wheel electric motors. The third model, the Venture EV will be all-electric. The estimated $20K-ish starting price tag and the trio’s expansive performance portfolio –120mph for the Q100, a 350-mile range for the e50, and 75mph and a 120-mile range for the EV – promises every John G. Public a little something. But this is a sexiness competition after all, and it’s the use of Carver Engineering’s Dynamic Vehicle Control system that pushes the VentureOne to the top of this conceptual dogpile of the sexy.